1. Field of the Invention
The present invention generally relates to reticles used in photolithographic semiconductor manufacturing and more particularly to an improved reticle that includes a planar anti-reflective surface which is easily checked for foreign matter.
2. Description of the Related Art
Transparent pellicles are used to protect the surface of a reticle from the deposition of contaminants after it has been fabricated and while it is in use during the manufacture of semiconductor integrated circuits (for example, see U.S. Pat. No. 6,284,417, incorporated herein by reference). The requirement of a pellicle presents challenges to define new, cost effective materials for use at shorter imaging wavelengths. Currently, there are no known materials for use at 157 nm and shorter wavelengths, and at 193 nm pellicle durability is questionable. The ability to overcome this requirement, omit the pellicle and use a reticle without it, is precluded by the inability to rapidly and adequately inspect and remove deposited contaminants and ensure defect free integrated circuit imaging.
After a reticle has been fabricated and is in use in the manufacture of integrated circuits, there are two conventional methods that can be utilized to qualify the reticle as defect free. The first method is to perform a thru-the-reticle transmission and reflectance inspection test against a reticle design database, and/or identical reticle pattern within the same reticle or another reticle. The second method is to image a wafer with the reticle and inspect the wafer image against a reticle design database, and/or identical pattern within the same reticle pattern area or generated with another reticle.
The first method requires expensive equipment and very long inspection times. It provides sufficient contaminant detection capability and is currently used to initially qualify a reticle after it has been fabricated. However, it does not provide a timely or cost effective solution for a pellicle-less reticle operation, as the reticle would be required to be inspected very frequently. Possibly as often as every wafer. With wafer processing times on the order of 30 to 90 seconds per wafer, it is not cost effective to inspect a reticle for several hours between each wafer.
The second method has similar cost and time constraints. It is further inhibited by the low contrast of patterns on the wafer which limit defect detection sensitivity. This method also requires significant computer modeling to overcome the process induced changes (e.g., from chemical processing, lens aberrations, etc) when the reticle pattern is imaged onto a wafer. The additional loss of performance for inspection capability coupled with poor cost and lengthy inspection times result in an ineffective solution for pellicle-less operation of a reticle.
The use of other techniques within the exposure tool, such as grazing incidence (scattered light) inspection technology, are fast and cost effective, but are limited to large contaminants (much larger than the reticle design feature size) and are not able to qualify the conventional reticle as defect free. This is due to limits in the ability to distinguish between feature edges and contaminants, as well as not detecting contaminants in the recesses of the reticle topography. As a result, they do not represent a viable technique for pellicle-less operation with a conventional reticle. However, they are used very effectively, when inspecting conventional reticles, to capture contaminants that are deposited on a pellicle surface, due to the pellicle's flatness and its reflective nature to grazing incidence light.
When the conventional pellicle technique is not available (e.g., due to lack of acceptable materials of construction) or the pellicle solution is no longer cost effective as the wavelength of lithography source gets shorter, pellicle-less operation can be usable with cost effective particle detection and removal techniques provided.
Since pellicle-less reticles are likely for wavelengths at 157 nm and below, where currently there are no acceptable materials of construction or cost effective pellicle solutions, there is a need for an improved reticle structure that allows easy detection of foreign matter particles, without requiring a pellicle.